The following relates to non-invasive techniques for quantifying blood volume. It finds particular application to quantifying microvascular blood volume through magnetic resonance imaging techniques.
Conventionally, invasive and non-invasive techniques are used to measure blood volume. Invasive techniques include positron emission tomography (PET), dynamic contrast magnetic resonance imaging (DC MRI), dynamic susceptibility contrast MRI (DSC MRI), near infrared spectroscopy (NIRS), and optical imaging. Typical PET techniques are based on the administration of radioactive compounds, for example, through an arterial line or inhalation of C-11 or O-15 carbon monoxide. MRI techniques usually employ exogenous contrast agents (e.g., Gd-DTPA, MION) and a venous catheter for contrast agent injection/infusion.
One non-invasive approach uses nulling of the blood signal to measure blood volume changes. This approach is referred to as vascular space occupany (VASO). The VASO technique leverages the difference in longitudinal relaxation time T1 between blood and tissue and separates out the blood contributions to the parenchymal MR signal using a suitable inversion recovery time. Another non-invasive approach is blood-oxygen-level-dependent (BOLD) functional MRI (fMRI). BOLD fMRI depends on blood volume, blood relaxation times R2, etc., and could, in principle, be used to determine blood volume. For instance, BOLD fMRI potentially can be used to measure blood volume through established procedures involving hypoxia, hypercapnia, etc. However, the BOLD effect depends on many physiological parameters that render the derived blood volume changes sensitive to many inherent assumptions and calibrations.
In view of the aforementioned deficiencies of conventional blood volume quantifying techniques, there is an unresolved need for improved non-invasive blood volume quantifying techniques.